Rownum greater than query

Question

Hi all,
first of all, what I am aware about...
I am aware about the fact, that ROWNUM filters, inside the query which is to be filtered by rownum, are incorrect. That is this is incorrect:
select * from <table> where rownum < 20;
Also I am aware, that the following will never return a result:
select * from <table> where rownum > 20;
That is clear. The correct version of the "less than" query is:
select * from (select * from <table> order by a) where rownum < 20;
Now the question: what does the last example still not work with "greater than" queries:
Take e.g. following example:
select * from (
select * from dba_objects order by object_name
) where rownum > 5
;

Answer 1

I ckeck it again. It's working for me !
Any way I'll quote it.
How ROWNUM Works
ROWNUM is a pseudocolumn (not a real column) that is available in a query. ROWNUM will be assigned the numbers 1, 2, 3, 4, ... N, where N is the number of rows in the set ROWNUM is used with. A ROWNUM value is not assigned permanently to a row (this is a common misconception). A row in a table does not have a number; you cannot ask for row 5 from a table—there is no such thing.
Also confusing to many people is when a ROWNUM value is actually assigned. A ROWNUM value is assigned to a row after it passes the predicate phase of the query but before the query does any sorting or aggregation. Also, a ROWNUM value is incremented only after it is assigned, which is why the following query will never return a row:
select *
from t
where ROWNUM > 1;
Because ROWNUM > 1 is not true for the first row, ROWNUM does not advance to 2. Hence, no ROWNUM value ever gets to be greater than 1. Consider a query with this structure:
select ..., ROWNUM
from t
where <where clause>
group by <columns>
having <having clause>
order by <columns>;
Think of it as being processed in this order:
1. The FROM/WHERE clause goes first.
2. ROWNUM is assigned and incremented to each output row from the FROM/WHERE clause.
3. SELECT is applied.
4. GROUP BY is applied.
5. HAVING is applied.
6. ORDER BY is applied.
That is why a query in the following form is almost certainly an error:
select *
from emp
where ROWNUM <= 5
order by sal desc;
The intention was most likely to get the five highest-paid people—a top-N query. What the query will return is five random records (the first five the query happens to hit), sorted by salary. The procedural pseudocode for this query is as follows:
ROWNUM = 1
for x in
( select * from emp )
loop
exit when NOT(ROWNUM <= 5)
OUTPUT record to temp
ROWNUM = ROWNUM+1
end loop
SORT TEMP
It gets the first five records and then sorts them. A query with WHERE ROWNUM = 5 or WHERE ROWNUM > 5 doesn't make sense. This is because a ROWNUM value is assigned to a row during the predicate evaluation and gets incremented only after a row passes the WHERE clause.
Here is the correct version of this query:
select *
from
( select *
from emp
order by sal desc )
where ROWNUM <= 5;
This version will sort EMP by salary descending and then return the first five records it encounters (the top-five records). As you'll see in the top-N discussion coming up shortly, Oracle Database doesn't really sort the entire result set—it is smarter than that—but conceptually that is what takes place.